The selective oligomerization of ethylene provides a vital route to the production of specific linear alpha-olefins (LAO), and is an important technological research task in olefin polymerization and catalysis field. Among the linear alpha-olefins, 1-hexene and 1-octene have become important comonomer for production of linear low density polyolefin (LLDPE). Compared with polyethylene, multiple performances of the polyolefin products, which are produced with 1-hexene and 1-octene as the comonomer, can be significantly improved, such as mechanical performance, optical performance, tearing resistance strength, and impact resistance strength. The polyolefin products can be used as film materials, and have a wide use and a large demand in industrial production.
In traditional ethylene oligomerization reaction, aluminum-based catalyst, titanium-based catalyst, nickel-based catalyst, zirconium-based catalyst, iron-based catalyst, or cobalt-based catalyst are used. The reaction mainly follows the Cossee-Arlman mechanism. That is, ethylene proceeds with linear chain-growth in the presence of the metal catalyst, and then proceeds with chain-elimination by β-hydrogen migration, thereby generating linear alpha-olefins. During the reaction procedure, the chain-growth rate is slightly larger than or roughly equal to the chain-elimination rate. Therefore, the carbon chain grows within a certain limit, and the number of carbon atoms mainly ranges from 4 to 30, and the alpha-olefin products generated therein are in consistent with Schulz-Flory or Poisson distribution. The content of 1-hexene and 1-octene in the products is relatively low (J. Organomet. Chem. 2004, 689, 3641). U.S. Pat. No. (3,676,523) discloses a Shell Higher Olefin Process (SHOP) for producing alpha-olefins using nickel-based catalyst, and the content of C6 to C10 olefins in the alpha-olefin products ranges from 21.0% to 52.0%. A oligomerization product under a catalytic effect of an iron-based catalyst is reported by Brookhart et al, wherein the content of C6 and C8 olefins ranges from 47% to 52% (J. Am. Chem. Soc., 1998, 120, 7143; U.S. Pat. No. 6,103,946).
The selective oligomerization reaction of ethylene is a subsequently developed method, which has a very important application prospect and an important role in industrial synthesis of specific linear alpha-olefins. The production of 1-hexene through selective trimerization of ethylene is firstly discovered by Union Carbide Corporation (U.S. Pat. No. 3,300,458). In 1999, the catalytic reaction for ethylene trimerization using a system of 2-ethylhexanoate chromium, 2,5-dimethylpyrrole, triethylaluminum, and diethylaluminum chloride in cyclohexane solvent under a temperature being 115° C. and a pressure being 100 bar is reported by Phillips. The reaction has a high selectivity, 1-hexene accounts for 93% of the products, and an activity of the catalyst reaches 1.56×105 g/(g Cr·h) (U.S. Pat. No. 5,856,612). In 2002, the catalytic reaction for trimerization of ethylene using a system of chromium trichloride, PNP-ligand, and methylaluminoxane in toluene solvent under a temperature being 80° C. and a pressure being 20 bar is reported by British Petroleum (BP). The reaction also has a high selectivity, 1-hexene accounts almost for 90% of the products, and an activity of the catalyst reaches 1.03×106 g/(g Cr·h). Besides, the reaction condition is relatively mild (Chem. Commun. 2002, 858; U.S. Pat. No. 5,856,612). In 2004, based on the research of British Petroleum, the aryl substituent on the phosphorus atom of the PNP-ligand, i.e., the methoxyl is replaced to be H or alkyl group by researchers of Sasol limited, and the new ligand forms a system with chromium trichloride and methylaluminoxane. The catalytic reaction for ethylene tetramerization is then performed using the aforesaid system in toluene solvent under a temperature being 65° C. and a pressure being 30 bar or under a temperature being 45° C. and a pressure being 45 bar. 1-octene accounts for 70% of the products, and an activity of the catalyst ranges from 8.05×103 g/(g Cr·h) to 4.36×104 g/(g Cr·h). Then, various replacements of substituents on phosphorus atom and nitrogen atom are performed, and the catalytic results of selective tetramerization of ethylene obtained therein are similar to the above result (J. Am. Chem. Soc., 2004, 126, 14712; WO 056478).
The catalytic reaction mechanism of selective trimerization of ethylene is different from the Cossee-Arlman mechanism of linear chain growth. The catalytic reaction for selective trimerization of ethylene mainly follows a metal cyclization reaction mechanism. That is, ethylene molecules are trimerized in the presence of the metal catalyst to form a CrC6 seven-membered ring intermediate compound, and then form hexane after intra-ring β-H migration and reduction elimination reaction. During selective tetramerization of ethylene, an ethylene molecule is inserted into the CrC6 seven-membered ring intermediate so as to form a CrC8 nine-membered ring intermediate compound, and then form octane after intra-ring β-H migration and reduction elimination reaction. The selective tetramerization of ethylene is relatively difficult, because the CrC8 nine-membered ring intermediate has a poor structural stability compared with the CrC6 seven-membered ring intermediate. However, the selective tetramerization of ethylene can be realized through electronic and steric structural regulation on the ligand around Cr of the catalyst, which is fully recognized by the researchers in the art. Nevertheless, related work is done very little.
At present, the industrial directional production of 1-hexene is realized by selective trimerization of ethylene. For example, Phillips successfully built a 1-hexene production equipment in Qatar during 2003. The product of this equipment is single, and the selectivity of 1-hexene reaches 90% or above. Besides, the production equipment has less by-products, a high catalytic activity, and a simple work flow. The 1-hexene production equipment through selective trimerization of ethylene in Daqing Petrochemical Company in China is already put into production, and the similar equipment in Yanshan Petrochemical Company in China is about to be put into production. The industrial production of 1-octene is about to be realized by selective tetramerization of ethylene, but there are still theoretical and technological difficulties in this respect.